新疆北山红石山含铜镍镁铁-超镁铁质岩体PGE和Re-Os同位素地球化学特征及成矿意义

对新疆北山地区红石山镁铁-超镁铁质岩体中含铜镍的硫化物矿石和岩石进行了铂族元素和Re-Os同位素地球化学特征研究,结果表明,矿石及岩石的铂族元素(PGE)总量较低,变化于0.54×10-9~15.84×10-9之间。较低的Pd/Ir比值表明岩石主要受岩浆作用控制,后期热液作用影响不明显。较高的Cu/Pd和Ti/Pd比值表明岩浆在演化过程中发生了硫化物的熔离。岩体的母岩浆为有早期结晶橄榄石加入的高镁的玄武质岩浆。γOs(t)的变化较大,变化于-282~+282之间,表明有较多的地壳物质混入。地壳物质混染和橄榄石等矿物的分离结晶可能是引起岩浆中的S达到饱和进而熔离的重要因素。红石山岩体是经历了结晶分异和硫化物熔离后橄榄石的堆积体与残余岩浆演化的混合体。     

Re–Os isotope and platinum-group element geochemistry of the Pobei Ni–Cu sulfide-bearing mafic–ultramafic complex in the northeastern part of the Tarim Craton

A number of mafic–ultramafic intrusions that host Ni–Cu sulfide mineralization occur in the northeastern Tarim Craton and the eastern Tianshan Orogenic Belt (NW China). The sulfide-mineralized Pobei mafic–ultramafic complex is located in the northeastern part of the Tarim Craton. The complex is composed of gabbro and olivine gabbro, cut by dunite, wehrlite, and melatroctolite of the Poyi and Poshi intrusions. Disseminated Ni–Cu sulfide mineralization is present towards the base of the ultramafic bodies. The sulfide mineralization is typically low grade (<0.5 wt.% Ni and <2 wt.% S) with low platinum-group element (PGE) concentrations (<24.5 ppb Pt and <69 ppb Pd); the abundance of Cu in 100 % sulfide is 1–8 wt.%, and Ni abundance in 100 % sulfide is typically >4 wt.%. Samples from the Pobei complex have εNd (at 280 Ma) values up to +8.1, consistent with the derivation of the magma from an asthenospheric mantle source. Fo 89.5 mol.% olivine from the ultramafic bodies is consistent with a primitive parental magma. Sulfide-bearing dunite and wehrlite have high Cu/Pd ratios ranging from 24,000 to 218,000, indicating a magma that evolved under conditions of sulfide saturation. The grades of Ni, Cu, and PGE in 100 % sulfide show a strong positive correlation. A model for these variations is proposed where the mantle source of the Pobei magma retained ~0.033 wt.% sulfide during the production of a PGE-depleted parental magma. The parental magma migrated from the mantle to the crust and underwent further S saturation to generate the observed mineralization along with its high Cu/Pd ratio at an R-factor varying from 100 to 1,200. The mineralization at Poshi and Poyi has very high γOs (at 280 Ma) values (+30 to +292) that are negatively correlated with the abundance of Os in 100 % sulfide (5.81–271 ppb) and positively correlated with the Re/Os ratios; this indicates that sulfide saturation was triggered by the assimilation of crustal sulfide with both high γOs and Re/Os ratios. When compared to other Permian mafic–ultramafic intrusions with sulfide mineralization in the East Tianshan, the Poyi and Poshi ultramafic bodies were formed from more primitive magmas, and this helps to explain why the sulfide mineralization has high Ni tenor.     

塔里木板块东北部坡一镁铁质-超镁铁质层状侵入体岩石成因

坡一侵入体位于塔里木板块东北部坡北岩体内,是该岩体第三阶段岩浆活动形成的十几个小侵入体中的一个,锆石U-Pb年龄为278±2Ma。该侵入体属于以超镁铁质岩石为主的层状岩系,堆晶结构与韵律性堆晶层理非常发育。岩浆分异充分,形成了从纯橄岩到石英闪长岩的多种岩石类型。在超镁铁质岩石中,所有的橄榄石和大部分斜方辉石是堆晶相,少量斜方辉石是填隙相,大部分单斜辉石、褐色普通角闪石和黑云母是填隙相。在镁铁质岩石中,橄榄石和斜方辉石全部是堆晶相,单斜辉石与斜长石既可以是堆晶相,也可以是填隙相;褐色普通角闪石、黑云母和石英均为填隙相。超镁铁质岩石属拉斑玄武岩系列,镁铁质岩石属钙碱性系列。侵入体中大量存在的捕掳体、微量元素地球化学、Nd-Sr同位素组成的EMⅡ型演化趋势,充分证明了同化混染作用伴随岩浆演化过程而逐渐增强,并不断促进了岩浆的分异,而且导致了岩石化学系列的转化。PGE和亲硫元素地球化学以及硫同位素组成证明,硫主要来自于岩浆,硫化物形成于岩浆阶段,岩浆未经历过早期硫化物熔离作用,硫化物熔离起始于橄榄岩相结晶的晚期阶段,并伴随着此后的岩浆演化过程而继续熔离。硫化物熔离是岩浆自身演化和同化混染共同作用的结果。橄榄石Fo分子含量和全岩FeO含量显示,原生岩浆是苦橄质岩浆;源区物质应该是石榴石辉石岩;岩浆生成于地幔柱轴部。在塔里木板块东北部还存在分别来自于软流圈和亏损型大陆岩石圈地幔的二叠纪岩浆岩,它们都应该是塔里木大火成岩省的组成部分。     

The Gaojiacun Mafic-Ultramafic Complex （Sichuan, SW China）：Neoproterozoic Magmatic Activity at the Western Margin of the Yangtze Craton

1 Introduction The Gaojiacun intrusive complex is one of the numerous ultramafic-mafic intrusions in Sichuan Province of China. It was mapped during the 1970s and studied mainly by Chinese scientists (e.g. Geological Team 106, 1975; Shen et al., 1986, 1989; CGGJC, 1986; Yang et al., 1993; Li et al., 1995; Shen et al., 2003; Zhu et al., 2004a). Since the year 2000, China has become one of the largest PGE consumers. While the country can produce only less than 1 ton PGE/year, the Chin…     

新疆北山地区罗东镁铁质-超镁铁质层状岩体岩石成因

罗东镁铁质-超镁铁质岩体位于塔里木板块东北部的新疆北山地区,岩体平面形态为眼球状,出露面积约2.1 km2.由纯橄岩、单辉橄榄岩、斜长二辉橄榄岩、橄榄二辉岩、方辉辉石岩、橄长岩、橄榄辉长岩、辉长岩、苏长辉长岩和淡色辉长岩组成,堆晶结构和堆晶韵律发育,属于层状岩体.岩浆演化过程中主要分离结晶/堆晶相是橄榄石和单斜辉石,此...     

牛鼻子梁镁铁质-超镁铁质杂岩体岩石特征

牛鼻子梁岩体位于柴达木地块的北缘,出露面积约8 km²,平面形态呈长条状,主要由斜长二辉橄榄岩、斜长单辉橄榄岩、角闪二辉橄榄岩、角闪橄榄岩、角闪橄榄二辉岩、黑云母化二辉岩、角闪辉石岩、橄榄辉石角闪石岩、角闪橄榄辉长岩、细粒辉长岩、似斑状辉长岩、暗色辉长岩、辉长岩、淡色辉长岩、石英闪长岩和英云闪长岩组成。文章通过岩石学、矿物学、地球化学研究,得到锆石U-Pb年龄为（361.5±1.2） Ma,Sm-Nd等时线年龄为（347±26） Ma。研究认为,牛鼻子梁基性-超基性岩体含矿岩石产于大陆边缘环境。岩体形成于泥盆纪晚期。岩浆分异充分,岩石类型丰富,岩浆演化过程中主要发生了橄榄石和斜长石的分离结晶/堆晶作用。岩体的母岩浆应属于拉斑玄武岩质岩浆。从目前发现的矿化情况来看,牛鼻子梁基性-超基性杂岩体为含矿岩体,有很好的找矿前景。     

Compositional variations of several Early Permian magmatic sulfide deposits in the Kalatongke district,southern Altai,western China: With genetic and exploration implications

A Permian magmatic Ni-Cu sulfide deposit cluster occurs in the Kalatongke district in the Southern Chinese Altai Orogenic Belt, western China. These deposits are associated with the mafic units of the Y1, Y2, Y3, Y9 and G21 mafic-intermediate complexes. In this paper we report the first zircon U-Pb ages for the Y3 and G21 intrusions, which are 283.3 ± 1.3 Ma and 281.1 ± 1.5 Ma, respectively. Our new age data confirm that the sulfide-bearing mafic units of the Y1, Y2 (connected with Y1 at depth), Y3, Y9 and G21 intrusions all formed in Early Permian between ∼281 and ∼287 Ma. New and existing petrological-geochemical data show some important regular variations between these deposits. The host lithologies change from olivine-bearing rocks for the Y1-Y2-Y9 deposits to olivine-free rocks such as norite for the Y3 deposit and leucogabbro for the G21 deposit. The olivine Fo contents of the Y1 deposit are up to 82 mol%, which are slightly higher than those of the Y2 deposit (up to 81 mol%) and the Y9 deposit (up to 79 mol%). The average plagioclase An contents of the olivine-bearing Y1-Y2-Y9 deposits are higher than those of the olivine-free Y3-G21 deposits. Among the three deposits (Y1, Y2 and Y3) that occur closely along the same structural lineament, the Ni/Cu ratios of bulk sulfides decrease from the olivine-bearing deposits (Y1 and Y2) to the olivine-free deposit (Y3). The PGE tenors of these deposits (Y1, Y2 and Y3) and the nearby coeval deposits (Y9 and G21) are extremely low, indicating that their parental magmas are severely depleted in PGEs. The variations of PGE tenors within a single deposit as well as among the different deposits are mainly due to variable R factors. The host rocks of these deposits are all characterized by elevated initial ⁸⁷Sr/⁸⁶Sr ratios from 0.7045 to 0.7047, positive εNd values from 4.95 to 6.86, positive εHf values of zircon from 9 to 16, and elevated δ¹⁸O values of zircon from 6.15 to 6.7‰. The isotope data indicate that the parental magmas for these deposits experienced up to ∼15 wt% crustal contamination. The δ³⁴S values of the sulfide minerals from these deposits are from −3.1‰ to 0.4‰, with a peak at −2.2‰, indicating the involvement of crustal sulfur. The isotope data and mineral chemistry together indicate that both olivine fractional crystallization and addition of crustal sulfur played a role in triggering sulfide saturation in the parental magmas for these deposits. Based on higher Ni/Cu ratios of sulfide mineralization in the olivine-bearing intrusions (Y1, Y2, Y9) than in the coeval olivine-free intrusions (Y3, G21), we recommend that Ni exploration in the region focus on the olivine-bearing intrusions that were emplaced in the Early Permian.     

东昆仑石头坑德镁铁-超镁铁质岩体矿物学特征及成矿指示

石头坑德镁铁-超镁铁质岩体位于东昆仑造山带东段五龙沟地区,邻近昆中大断裂,主要岩性为辉石岩、橄辉岩、橄榄岩、辉长岩等,铜镍矿体主要赋存于Ⅰ号岩体的辉石岩、橄辉岩和橄榄岩中。岩石地球化学特征表明,其镁铁比值(m/f)变化范围在2.77~6.01之间,属铁质系列的镁铁-超镁铁岩,有利于成铜镍矿。稀土总量总体较低,轻稀土元素之间分馏强,重稀土元素之间分馏弱。岩石总体富集LILE(Rb、Ba、Th、U),贫HFSE(Nb、Ta、Zr、Y),具有明显的Rb、U正异常及Nb、Ta的负异常。超镁铁岩类橄榄石中的Ni普遍亏损,指示深部存在硫化物的熔离作用,在橄榄石结晶之前,大量的Ni进入到硫化物中,极有利于形成富矿岩浆或矿浆。可见,伴随同化混染作用的发生,岩浆中S达到过饱和,进而硫化物发生不混溶作用,富含硫化物的岩浆运移至东昆仑造山带邻近昆中断裂构造薄弱的部位成岩成矿。铜镍矿体多赋存在含橄榄石的超镁铁质岩体内,指示石头坑德岩体深部存在高度富集铜镍的部位,是下一步的找矿方向。     

Formation of plagioclase-bearing peridotite and plagioclase-bearing wehrlite and gabbro suite through reactive crystallization: an experimental study

Plagioclase-bearing peridotites are commonly associated with gabbroic rocks sampled around the Moho Transition Zone. Based on mineral chemistry, texture, and spatial relations, the formation of plagioclase-bearing peridotites has been attributed to impregnation of basalt into residual peridotites. We conducted reactive dissolution and crystallization experiments to test this hypothesis by reacting a primitive mid-ocean ridge basalt with a melt-impregnated lherzolite at 1,300 °C and 1 GPa and then cooling to 1,050 °C as pressure decreased to 0.7 GPa. Crystallization during cooling produced lithologic sequences of gabbro–wehrlite or gabbro–wehrlite–peridotite, depending on reaction time. Wehrlitic and peridotitic sections contain significant amounts of plagioclase interstitial to olivine and clinopyroxene and plagioclase compositions are spatially homogeneous. Clinopyroxene in the wehrlite–peridotite section is reprecipitated from the melt and exhibits poikilitic texture with small rounded olivine chadacrysts. Mineral composition in olivine and clinopyroxene varies spatially, both at the scale of the sample and within individual grains. Olivine grains that crystallized close to the melt–peridotite interface are enriched in iron due to their proximity to the basaltic melt reservoir. Consistent with many field studies, we observed gradual spatial variation in olivine and clinopyroxene composition across a lithologically sharp boundary between the gabbro and wehrlite–peridotite. Plagioclase compositions show no obvious dependence on distance from the melt–rock interface and were precipitated from late-stage trapped melts. Compositional trends of olivine, pyroxene, and plagioclase are consistent with previous experimental results and natural observations of the Moho Transition Zone. Different lithological sequences form based primarily on the melt–rock ratio, composition of the melt and host peridotite, and thermochemical conditions, but are expected to grade from gabbro to wehrlite or troctolite to peridotite. Plagioclase-bearing peridotite represents the low melt–rock ratio end member where pyroxene is only partially replaced by olivine and melt, whereas dunite is expected to form where melts overwhelm and consume all other phases. This study confirms that under nominally anhydrous conditions, the gabbro–wehrlite–plagioclase-peridotite sequence can be formed by reaction between basalt and lherzolite and subsequent crystallization at intermediate to low pressures. Melt–rock reaction is a fundamental process in the formation of new crust at the shallowest part of the melting column where pyroxene-undersaturated melts percolate through depleted peridotite.     

Magmatic modification of the uppermost mantle beneath the Basin and Range to Colorado Plateau Transition Zone; Evidence from xenoliths, Wikieup, Arizona

Upper mantle xenoliths from Wikieup, AZ, provide abundant evidence for magmatic modification of the uppermost mantle beneath the Transition Zone between the Colorado Plateau and the southern Basin and Range province. Upper mantle lithologies in this xenolith suite are represented by spinel peridotite, wehrlite, plagioclase peridotite, and Al-augite group pyroxenites. Isotopic data for these xenoliths yield relatively uniform values and suggest a common petrogenesis. Al-augite-bearing gabbro and pyroxenite xenoliths from this locality are interpreted to have formed by crystal fractionation processes from parent alkali basalts similar to the Wikieup host basalt. Mineral and whole rock compositions show consistent trends of increasing incompatible element contents (Fe, Al, Ca, Na, K, LIL, and LREE), and decreasing compatible element contents (Mg, Cr, Ni) from spinel peridotite to wehrlite to plagioclase peridotite to the host basalt composition. These compositional trends are interpreted as resulting from varying degrees of magma-mantle wall rock interaction as ascending mafic magmas infiltrated upper mantle peridotite. Small degrees of melt infiltration resulted in slightly modified spinel peridotite compositions while moderate degrees metasomatized spinel peridotite to wehrlite, and the highest degrees metasomatized it to plagioclase peridotite. Whole rock compositions and clinopyroxene, plagioclase, and whole rock isotopic data suggest that the infiltrating magmas were the same as those from which the gabbros and pyroxenites crystallized, and that they were alkalic in composition, similar to the Wikieup host alkali olivine basalts. Relatively uniform ¹⁴³Nd/¹⁴⁴Nd for the mineral separates and whole rocks in spite of the significantly wide range in their ¹⁴⁷Sm/¹⁴⁴Nd (0.71–0.23 in clinopyroxene) suggests that the Wikieup xenoliths including gabbro, pyroxenite, peridotite, wehrlite, and plagioclase peridotite, are all relatively young rocks formed or metasomatized by a relatively recent magmatic episode. Received: 21 May 1996 / Accepted: 23 December 1996     

东天山图拉尔根铜镍钴硫化物矿床岩相、岩石地球化学特征及其形成的构造背景

新疆图拉尔根铜镍钴矿产于康古儿塔格—黄山韧性剪切带的北东段,是由硫化物深部熔离成矿为主兼就地熔离、热液叠加成矿多重作用形成的半隐伏矿床。1号岩体以全岩矿化为特征,可分为4个岩相:角闪橄榄岩、辉石橄榄岩、角闪辉石岩、辉长岩。岩性具有单期岩浆多次脉动上涌成矿特征。岩体m/f值为3.1～4.8,属于铁质超镁铁岩类,且具有低钛、低碱、低Al2O3特征,与黄山—镜儿泉镁铁质-超镁铁质杂岩带岩石化学特征相似。由稀土元素配分曲线和微量元素、过渡族元素蛛网图可知1号和2号岩体具有同源性,并具有互补性,预示2号岩体深部成矿潜力很大,虽然其地表辉长岩矿化微弱。根据横穿1号和2号岩体的大地电磁测深剖面图可以看出两个岩体在深部具有同一个岩浆通道,也验证了两岩体属于同一岩浆来源。较低的La/Sm(<2)和Th/Ta值(4.6)表明成矿岩浆为地幔来源,岩体就位时很少受到地壳的混染。     

东天山图拉尔根大型铜镍硫化物矿床的斜长石主量成分特征：对矿床成因的指示

图拉尔根镍铜矿床产于康古尔塔格-黄山韧性剪切带NEE向的次级挤压破碎带上,位于黄山-镜儿泉岩浆铜镍成矿带的东段。矿田范围内有I、II、III号3个镁铁质-超镁铁质杂岩体,主要铜镍矿体就位于I号基性-超基性杂岩体内。该杂岩体主要包括辉长岩、角闪橄榄岩、二辉橄榄岩、角闪辉橄岩等岩相,其中角闪橄榄岩为主要的赋矿岩相。本文通过对矿物颗粒镜下形态、结构、构造的观察,对钻孔中赋矿岩相、不含矿岩相的斜长石进行系统的成分分析和对比,来进一步限定岩浆的演化及矿床成因。图拉尔根矿床中主要造岩矿物的结晶次序为:橄榄石-辉石(角闪石)-角闪石。仅见极少数斜长石颗粒包裹橄榄石,而与辉石和角闪石没有明显的包裹关系,可能是略晚于橄榄石结晶的另一单独结晶序列的矿物。赋矿的角闪橄榄岩中斜长石与硫化物含量呈负相关性。斜长石中SiO2和Al2O3含量相对集中,CaO、Na2O、K2O含量变化范围大。SiO2含量从深部到浅部,表现出含量从低到高变化的趋势,说明岩体中斜长石受同源岩浆结晶分异作用的控制。越靠近富硫化物的角闪橄榄岩,斜长石中CaO和Al2O3含量越高,SiO2、Al2O3、CaO、Na2O、K2O等主量元素的含量变化范围较大。因此,斜长石的主量成分可以作为接近硫化物富矿体的指示。     

Genesis of the Huangshannan high-Ni tenor magmatic sulfide deposit in the Eastern Tianshan,northwest China: Constraints from PGE geochemistry and Os–S isotopes

The Huangshannan magmatic Ni-Cu sulfide deposit is one of a group of Permian magmatic Ni-Cu deposits located in the southern Central Asian Orogenic belt in the Eastern Tianshan, northwest China. It is characterized by elevated Ni tenor (concentrations in recalculated 100% sulfide) in sulfide within ultramafic rocks (9–19 wt%), with values much higher than other deposits in the region. Sulfides of the Huangshannan deposit are composed of pentlandite, chalcopyrite, and pyrrhotite and the host rock is relatively fresh, indicating that the high-Ni tenor is a primary magmatic feature rather than formed by alteration processes. It is shown that sulfides with high-Ni tenor can be generated by sulfide-olivine equilibrium at an oxygen fugacity of QFM +0.5, for magmas containing 450 ppm Ni and 20% olivine. Ores with >10 wt% sulfur have relatively low PGE and Ni tenors compared to other ores, R factor (mass ratio of silicate to sulfide liquid) modeling of Ni indicates that they formed at moderate R values (150–600). Based on this constraint on R values, ores with <10 wt% sulfides in the Huangshannan deposit can be segregated from a similar parental magma with 0.05 ppb Os, 0.023 ppb Ir, and 0.5 ppb Pd at R values between 600 and 3000. This, coupled with the supra-cotectic proportions of sulfide liquid to cumulus silicates in the Huangshannan ores imply mechanical transport and deposition of sulfide liquid in a magma pathway or conduit, in which sulfides must have interacted with large volumes of silicate magma. Platinum and Pd depletion relative to other platinum group elements (PGEs) are observed in fresh and sulfide-rich samples (S > 4.5 wt%). As sulfide-rich samples are also depleted in Cu, and as interstitial sulfides in those samples are physically interconnected at a scale of several cms, the low Pt and Pd anomalies are attributed to solid Pt and Pd phases crystallization and retention with the monosulfide solid solution (MSS) and Cu-rich sulfide liquid percolation during MSS fractionation. This finding indicates that Pt anomalies in sulfide-rich rocks from magmatic Ni-Cu deposits in the Eastern Tianshan are the result of sulfide fractionation rather than a hydrothermal effect. ¹⁸⁷Os/¹⁸⁸Os_(278Ma) values of the lherzolite samples vary from 0.27 to 0.37 and γOs_(278Ma) values vary from 110 to 189, indicating significant magma interaction with crustal sulfides, rich in radiogenic Os. Well constrained γOs values and δ³⁴S values (−0.4 to 0.8‰) indicate that crustal contamination occurred at depth before the arrival of the magma in the Huangshannan chamber. Regionally, deposits with high-Ni tenor have not been reported other than the Huangshannan deposit; however, many intrusions with high-Ni contents in olivine are present in NW China, such as the Erhongwa, Poyi and Poshi intrusions. Those intrusions are capable of forming high-Ni tenor sulfides due to olivine-sulfide-silicate equilibrium and relative high-Ni content in parent magma, making them attractive exploration targets.     

Geology,petrology and geochemistry of the “Americano do Brasil” layered intrusion,central Brazil,and its Ni–Cu sulfide deposits

The “Americano do Brasil” Complex (ABC) is part of a cluster of coeval synorogenic mafic–ultramafic intrusions emplaced during the Brasiliano/Pan-African Orogenic Cycle in Brazil. The medium-sized ABC consists of interlayered dunite, peridotite, websterite, and gabbronorite. High Fo values of olivine (up to Fo₈₈) and the crystallization sequence of the ABC (Ol + Chr ≥ Ol + Opx + Chr ≥ Cpx + Opx ≥ Opx + Pl + Cpx ≥ Opx + Pl + Cpx + Ilm + Mag) suggest crystallization from tholeiitic high-MgO parental magmas. Light rare earth element (REE)-enriched mantle-normalized REE profiles and εNd(T) values of +2.4 for cumulate rocks from the ABC suggest a depleted mantle source for the parental magma. The ABC Ni–Cu sulfide deposit (3.1 Mt at 1.12 wt.% Ni and 1.02 wt.% Cu) consists of three distinctively different orebodies (S1, S2, and G2). The S2 orebody, an unusual occurrence of stratiform massive sulfide hosted by dunite and peridotite in the interior of a layered intrusion, results from sulfides accumulated at the transient base of the magma chamber following a new influx of parental magma. The G2 orebody has an irregular and roughly cylindrical shape, consisting mainly of net-textured sulfides. The G2 orebody is hosted by peridotite and pyroxenite and located stratigraphically below the S1 orebody. S2 and G2 orebodies are characterized by low Cu/Cu + Ni ratios (mainly below 0.4). The S1 orebody, hosted by websterite and gabbronorite in the more fractionated sequence of the ABC, is a cluster of several irregular discontinuous orebodies of Ni–Cu disseminated sulfides. The sulfides of the S1 orebody have high Cu/Cu + Ni ratios (mainly between 0.5 and 0.8) and are highly depleted in PGE. The S1 orebody is interpreted to result from a later event of sulfide segregation in the magma chamber, possibly following the event that originated the G2 orebody. The bulk of δ³⁴S values for sulfides of the ABC orebodies and their host rocks fall in the range of 0 ± 2‰. Higher δ³⁴S values (between 3‰ and 5‰) are restricted to pyrite from xenoliths of gneiss located close to the S1 orebody and sulfides from the S1 orebody. Crustal xenoliths and chemical data (lithogeochemistry and sulfur isotope composition) provide evidence of crustal contamination of the igneous rocks hosting the S1 orebody, suggesting that sulfur saturation was induced by contamination with sulfide-bearing crustal rocks. The ABC deposit is an example of Ni–Cu sulfide mineralization hosted by synorogenic mafic–ultramafic intrusions. The S2 orebody is the first documented example of an economic stratiform massive sulfide orebody located within layered intrusions, expanding the opportunities for exploration of Ni–Cu sulfides in orogenic regions worldwide.     

Geochemical and Petrological Studies on the Early Carboniferous Sidingheishan Mafic–Ultramafic Iintrusion in the Southern Margin of the Central Asian Orogenic Belt, NW China

The Sidingheishan mafic-ultramafic intrusion is located in the eastern part of the Northern Tianshan Mountain, along the southern margin of the Central Asian Orogenic Belt in northern Xinjiang autonomous region of China. The Sidingheishan intrusion is mainly composed of wehrlite, olivine websterite, olivine gabbro, gabbro and hornblende gabbro. At least two pulses of magma were involved in the formation of the intrusion. The first pulse of magma produced an olivine-free unit and the second pulse produced an olivine-bearing unit. The magmas intruded the Devonian granites and granodiorites.An age of 351.4±5.8 Ma(Early Carboniferous) for the Sidingheishan intrusion has been determined by U-Pb SHRIMP analysis of zircon grains separated from the olivine gabbro unit. A U-Pb age of 359.2±6.4 Ma from the gabbro unit has been obtained by LA-ICP-MS. Olivine of the Sidingheishan intrusion reaches 82.52 mole% Fo and 1414 ppm Ni. On the basis of olivine-liquid equilibria, it has been calculated that the MgO and FeO included in the parental magma of a wehrlite sample were approximately10.43 wt% and 13.14 wt%, respectively. The Sidingheishan intrusive rocks are characterized by moderate enrichments in Th and Sm, slight enrichments in light REE, and depletions in Nb, Ta, Zr and Hf. The ε_(Nd)(t) values in the rock units vary from +6.70 to +9.64, and initial ~(87)Sr/~(86)Sr ratios range between 0.7035 and0.7042. Initial ~(206)Pb/~(204)Pb, ~(207)Pb/~(204)Pb and ~(208)Pb/~(204)Pb values fall in the ranges of 17.23-17.91,15.45-15.54 and 37.54-38.09 respectively. These characteristics are collectively similar to the Heishan intrusion and the Early Carboniferous subduction related volcanic rocks in the Santanghu Basin, North Tianshan and Beishan area. The low(La/Gd)_(PM) values between 0.26 and 1.77 indicate that the magma of the Sidingheishan intrusion was most likely derived from a depleted spinel-peridotite mantle.(Th/Nb)_(PM)ratios from 0.59 to 20.25 indicate contamination of the parental magma in the upper crust.Crystallization modeling methods suggest that the parental magma of the Sidingheishan intrusion was generated by flush melting of the asthenosphere and subsequently there was about 10 vol%contamination from a granitic melt. This was followed by about 5 vol% assimilation of upper crustal rocks. Thus, the high-Mg basaltic parental magma of Sidingheishan intrusion is interpreted to have formed from partial melting of the asthenosphere during the break-off of a subducted slab.     

甘肃北山大山头南基性-超基性杂岩体SHRIMP锆石U-Pb定年及其地质意义

大山头南铜镍矿化基性-超基性杂岩体位于北山褶皱带东段,侵位于长城系古硐井群变质岩中。岩相分带明显,主要包括花岗闪长岩-闪长岩相、辉长岩-橄榄苏长辉长岩相和橄榄二辉岩-纯橄岩相。铜镍矿化主要赋存于橄榄二辉岩-纯橄岩相中。岩石化学特征显示K2O、Na2O、TiO2含量较低,Mg#值介于0.74~0.85之间,MgO/FeOT的值介于1.6~3.1之间,属于铁质镁铁-超镁铁岩;AFM图解显示该岩体具拉斑玄武岩的演化趋势;稀土元素总量较低,介于6.37×10-6~37.51×10-6之间;稀土元素标准化配分曲线表现为轻稀土元素稍富集的右倾型,LREE/HREE比值介于3.03~4.11之间,轻重稀土发生了明显的分馏;采用SHRIMP锆石U-Pb方法,获得杂岩体辉长岩中锆石U-Pb年龄为374.3±3 Ma(MSWD=0.92),表明该岩体侵位于中晚泥盆世,与形成大型镍矿床的黑山岩体年龄一致,形成于洋壳俯冲阶段。该年龄的获得不仅对北山乃至中亚造山带地质构造演化具有重要意义,而且也为北山褶皱带东段铜镍找矿提供了新的方向。     

高镍铜镍矿床的特征、形成机制与勘查展望

岩浆铜镍矿床100%硫化物中的Ni含量与赋矿岩石和成矿过程紧密相关,记录岩浆成分、分异程度与硫化物演化过程。硫化物异常高镍(高镍硫化物)往往被认为与科马提质岩浆或者后期热液作用密切相关。近年研究结合勘查证实,赋含高镍硫化物的矿床(高镍铜镍矿床)不仅限于科马提岩,还与苦橄质、玄武质岩浆有关,另外,热液富集作用并不是必要因素。本文总结了世界上高镍铜镍矿床的基本特征和形成机制,分析提出了不同机制的判别标志,并展望了其勘查前景。详细对比高镍铜镍矿床的产出环境、赋矿岩相、矿石特征、矿物组合等特征,该类矿床往往产于大陆裂谷和造山带环境,与基性程度较高的岩浆有关,以橄榄岩赋矿为主,含镍硫化物组合主要为镍黄铁矿-磁黄铁矿-黄铜矿组合,少数为针镍矿-镍黄铁矿-黄铁矿组合。科马提岩相关矿床可将Ni含量大于16%的硫化物定义为高镍硫化物,苦橄质-玄武质岩浆相关矿床的硫化物可分为高镍硫化物(Ni10%)、中镍硫化物(5%～10%)和富铜硫化物(Ni5%,CuNi)。原生高镍硫化物可由富镍岩浆熔离、硫化物从橄榄石中吸取Ni、硫化物结晶分异、硫化物与硫不饱和岩浆反应等机制形成。苦橄质-玄武质岩浆相关的矿床,硫化物与橄榄石的Fe-Ni交换反应是高镍硫化物形成的重要机制。辉石岩源区地幔部分熔融形成富镍岩浆是否为高镍硫化物形成的必要条件尚存争议。不同机制形成的高镍硫化物具有迥异的岩石-矿物组合和地化特征。硫化物矿物组合、橄榄石成分(Fo值、Ni含量、Fo值-Ni含量的相关性)、伴生元素(铜、铂族元素)丰度-配分模式等特征可作为区分不同高镍硫化物形成机制的有效指标。我国新疆黄山南、坡一和青海夏日哈木矿床(部分浸染状矿化橄榄岩)以赋含高镍硫化物为特征,新疆喀拉通克矿床的硫化物则以富铜为特征,中国其余矿床的硫化物均属中镍硫化物。目前研究指示中国的高镍铜镍矿床与母岩浆相对富镍、硫化物与橄榄石Fe-Ni交换作用密切相关,后者可使硫化物Ni含量提升3%～5%。在铜镍矿床勘查方面,稀疏-中等浸染状高镍硫化物矿石即可达到工业品位,稠密浸染状-块状高镍硫化物矿石可达到很高的Ni品位(10%),是高品位镍矿勘查的一个重要方向。造山带环境富水、相对高氧逸度(可高达QFM+1)的岩浆可能是形成高镍硫化物的有利条件,该环境橄榄石Fo值较高(87mol%)的岩体有利于形成高镍硫化物。     

阿拉斯加型岩体的基本特征、成岩过程及成矿作用

阿拉斯加型岩体是一类具有独特的岩性环带状结构的镁铁-超镁铁质侵入体,常呈链状分布于汇聚板块边缘。其形成时代跨度较大,从元古代到新生代均有分布,以中生代最为发育。大部分阿拉斯加型岩体规模较小,出露面积约12～14 km~2或更小,平面上呈近似同心环状结构,垂直剖面上呈管道状。岩体中心为纯橄岩,向外依次包括异剥橄榄岩、橄榄单斜辉石岩、单斜辉石岩、角闪单斜辉石岩、角闪石岩和辉长岩。造岩矿物为橄榄石、单斜辉石、角闪石等,副矿物为铬铁矿、磁铁矿、钛铁矿等,超镁铁质岩石中少或无斜方辉石,斜长石仅出现在边缘的辉长质岩石中。磁铁矿在单斜辉石岩和角闪石岩中为常见矿物,含量最高达15%～20%。阿拉斯加型岩体的主量元素成分揭示所有岩石均为与拉斑玄武质岩浆分异有关的亚碱性堆晶岩。微量元素成分上显示平坦的稀土元素配分型式和较低的微量元素含量,且富集大离子亲石元素,亏损高场强元素。矿物化学特征上,橄榄石富镁且Fo值变化较大;单斜辉石主要为富Ca的透辉石,其成分变化具有弧堆晶趋势;角闪石主要是镁角闪石和韭角闪石;铬铁矿富集Fe-Al,贫Cr。这些特征揭示,该类岩体成因明显不同于层状岩体和阿尔卑斯型岩体。综合岩石学、矿物学和地球化学分析表明,阿拉斯加型岩体形成于与板块俯冲作用有关的岛弧或者活动大陆边缘背景下,其母岩浆为受到熔/流体交代的地幔楔部分熔融产生的含水玄武质岩浆。各岩相为未受明显地壳混染的同源母岩浆在地壳深度结晶分异的产物。阿拉斯加型岩体的岩浆体系具有含水且高氧逸度的特征,其通常为铂族元素和铬铁矿矿床的重要载体,无或少铜镍矿化。     

阿拉善地块南缘野芨里镁铁-超镁铁质岩体锆石U-Pb年龄及其地质意义

阿拉善地块南缘北大山地区野芨里镁铁-超镁铁质岩体主要由单辉橄榄岩、二辉橄榄岩、橄榄辉长岩组成。岩体局部发现了硫化物,主要由磁黄铁矿、镍黄铁矿与黄铜矿组成,指示岩浆结晶过程中发生了硫化物不混溶作用。岩石地球化学分析显示野芨里岩体属于铁质超基性岩,富集大离子亲石元素,相对亏损高场强元素,具有明显的Nb,Ta负异常,微量元素比值指示岩浆在上升过程中受到了中上地壳物质混染。橄榄辉长岩中锆石SHRIMP U-Pb年龄为286.6±2.5 Ma,属于早二叠世。结合区域构造演化,认为野芨里岩体形成背景可能与古亚洲洋的闭合或俯冲之后的伸展作用相关。     

东天山四顶黑山层状岩体地质特征及成矿潜力分析

东天山黄山岩带因发育众多镁铁质-超镁铁质岩体和岩浆铜镍硫化物矿床而备受地质学者关注。四顶黑山岩体位于该岩带最东端,具有层状岩体特征;岩石类型有单辉橄榄岩、橄榄辉石岩、橄榄苏长辉长岩、橄榄辉长岩、辉石角闪岩、辉长岩、角闪辉长岩、闪长岩,主要造岩矿物为贵橄榄石、古铜辉石、透辉石、普通角闪石和斜长石,岩体分异较好,蚀变较发育。通过矿物显微结构特征观察以及计算得出:四顶黑山岩体中橄榄石最先结晶,其开始结晶温度大约在1419℃左右;古铜辉石和透辉石在橄榄石之后开始结晶,结晶温度分别在1100℃左右和900～1100℃之间,两矿物相在岩石中可以共存。岩体中橄榄岩相、苏长岩相发育,以及贵橄榄石+古铜辉石的矿物组合特征,表明岩体具有形成铜镍(铂)矿床的有利条件。